1. Field of the Invention
The invention relates to a circuit arrangement for generating an output signal at an output by combining first and a second input signal, comprising at least a controllable switches for applying a respective one of the input signals to the output, in which, for the purpose of switching the output from one of the input signals to the other, a first one of the controllable switches is gradually switched from the blocked to the conducting state, and a second one of the controllable switches is switched complementarily thereto from the conducting to the blocked state.
2. Description of the Related Art
German Patent DE 32 00 071 A1 corresponding to U.S. Pat. No. 4,430,609 discloses a signal transmission device with a transmission characteristic which is adjustable in steps, comprising at least a group of controllable switches for applying a signal at various inputs to an output. This signal transmission device comprises control means with which one of the switches is, each time, rendered conducting, these switches being rendered conducting and non-conducting by means of a control signal. A control signal generator is provided for generating a first control signal which changes uniformly with respect to time and, by means of which, a given switch of the afore-mentioned group of controllable switches is gradually rendered conducting, and for generating a second control signal which is complementary to the first control signal, this second control signal simultaneously and uniformly rendered, non-conducting another switch of this group. The described operation of switching the switches or changing the control signals may be performed stepwise.
German Patent DE 196 35 050 A1 corresponding to U.S. Pat. No. 5,963,081, discloses a circuit arrangement having at least two signal paths which can each be fed by a controllable current source and are alternately switched to activity by means of logic signals selecting one of the controllable current sources. When changing over from one signal path to another during a transition interval, the controllable current sources of the relevant two signal paths are to be controlled by a temporally constantly changing control signal in such a way that currents, which can be supplied by these two controllable current sources, change constantly and with opposite sense. In accordance with German Patent DE 196 35 050 A1, a device, with which a gradual transition from one adjusting position to the next is achieved, is embodied by a control circuit comprising a control stage for each signal path. Each control stage comprises an analog multiplex branch via which the current activating the controllable current source associated with the relevant signal path, can be derived from a reference current source comprised in the control circuit and being common for the multiplex branches of all control stages. Moreover, each control stage has a bistable stage for constant control of the current in the multiplex branch in dependence upon the control signal, and for storing the state of operation of the multiplex branch achieved at the end of the transition interval outside the transition interval, as well as a switching stage for switching the control signal during the transition interval by means of the logic signals via the bistable stage to the multiplex branch of that signal path which, after termination of the transition interval, is to be activated in dependence upon the logic signals.
In the known circuit arrangements, fading between the signals at the various outputs for forming the output signal is performed by way of an "analog" switching operation. In this analog switching operation, the control characteristic of the components used for switching is an essential factor of influence. Due to inaccuracies of these control characteristics, particularly due to tolerance deviations and inaccurate matching of the characteristics with each other, distortions may occur which become manifest during fading, particularly in the case of large signal differences. These distortions may noticeably affect the reproduction quality of the output signal. Particularly, when such a circuit arrangement is used for volume control in audio signal reproducing devices, these distortions may become audible and thus deteriorate the aural impression.